The hypothesis of the study is a 7.5% decrease of combined genito-urinary and intestinal side effects from 15% with photon therapy to 7.5% by the use of protons. With an alpha of 0.05 and 80% power it is therefore necessary to treat 131 patients with protons to detect such difference. With an estimated drop-out rate of 10% 146 patients have to be recruited to the proton arm. As randomization to photon or proton therapy is not possible in a multicenter setting with only few proton facilities available patients treated with photons will be prospectively matched to their proton counterparts. Stratification criteria will include: Age, high dose volume, androgen deprivation, history of surgery, obstructive disease, increased risk for rectal bleeding, socio-economic criteria and use of rectal balloon during radiotherapy. Comparison of the primary endpoint will be restricted to patients without irradiation of lymphatic drainage.